The present invention relates to a process for producing a powder of aluminum nitride, and in particular relates to a process for producing a powder of aluminum nitride having an average particle diameter of about 0.45 micron.
As it has a high thermal conductivity, an excellent electric insulating property, a coefficient of thermal expansion close to silicon single crystals, a low dielectric constant, the sintered aluminum nitride is a very suitable material for making printed circuit boards and heat release boards of the IC (integrated circuit), LSI (large scale integrated circuit) and VLSI (very large scale integrated circuit).
In general, two methods are used for the production of aluminum nitride powders. The first method is called direct nitriding method, which comprises nitriding metallic aluminum powder at a high temperature in an atmosphere of nitrogen or ammonia gas and pulverizing the resulting nitride. The second method is called the alumina reducing method, which comprises firing alumina and carbon powder in nitrogen or ammonia gas and pulverizing the resulting nitride.
Since metallic aluminum is used as a starting material in the first direct nitriding method, it naturally requires a step of pulverizing metallic aluminum in order to increase the rate of nitridation. Furthermore, to increase the sinterability of the resulting nitride, it requires a step of pulverizing the nitride to a particle size of less than several microns. Therefore, in the direct nitriding method, it is impossible to avoid the inclusion of impurities of metal or metallic compounds from pulverizing means such as a ball mill used in these pulverizing steps. In addition, because the direct nitriding method yields aluminum nitride powders inevitably containing impurities of unreacted metallic aluminum nitride, it is also extremely difficult to obtain aluminum nitride containing less than several percents by weight of impurities including those produced in the pulverizing steps. In the pulverizing step of the direct nitriding method, it is difficult to produce aluminum nitride powder having sufficiently small and uniform particle size. Moreover, since oxidation of the surface of the aluminum nitride powder during pulverization cannot be avoided, the aluminum nitride powder obtained by the direct nitriding method usually contains 2 to 5% by weight or even more of oxygen.
The second alumina reducing method is generally considered to be better than the direct nitriding method in that it gives an aluminum nitride powder having a relatively uniform particle size. But, to obtain particles having a size of less than several microns, the pulverizing step cannot be obviated. Furthermore, the content of the unreacted alumina cannot be reduced to an extreme extent. Accordingly, like the direct nitriding method, the second method also has the defect of yielding an aluminum nitride of low purity. Aluminum nitride powders produced by these methods do not have sufficient purity and generally assume a black to gray color. Therefore, as a general rule, not all sintered bodies obtained from these powders have light-transmitting properties.
In the Japan laid-open Patent Publication No. 59-207814 (Assignee: Tokuyama Soda Co., Ltd.), a process for producing a fine powder of alumina nitride is disclosed. According to the process disclosed in said publication, the produced aluminum nitride has an average particle diameter of less than 2 micron and a purity of at least 94 wt %, and the produced AlN powder can be used directly without further pulverizing steps. However, the average particle diameter of the produced AlN powder according to the process in said publication is not small enough, and the necessary amount of nitrogen gas used according to the process described in said publication is rather large, about 3000 cc/min.